Silicon photonics can potentially offer low-cost large-scale integration for photonics technology. Integration density is a key factor for its integration potential and economy of scale. Existing silicon photonics technology has not achieved integration density of one waveguide per micron. Although there exist approaches to high-density photonics integration at the expense of other performance metrics (e.g., Optical loss and/or crosstalk), a solution to high density photonics integration with minimal impact on other aspects of performance remain a central problem in practical photonic systems. Crosstalk between adjacent waveguiding components sets a fundamental limit for integration density. As the waveguide spacing decreases to the wavelength scale, crosstalk rises up exponentially. With high index contrast of silicon waveguides, while it is relatively easy to decrease the waveguide spacing to a few microns, below this limit every small decrease appears to be accompanied by intolerable surge of crosstalk.
Silicon photonics holds great promise for low-cost large-scale photonic integration. In its future development, integration density will play an ever increasing role in a way similar to that witnessed in integrated circuits. Waveguides are perhaps the most ubiquitous component in silicon photonics. As such, the density of waveguide elements is expected to have a crucial influence on the integration density of a silicon photonic chip. A solution to high-density waveguide integration with minimal impact on other performance metrics such as crosstalk remains a vital issue in many applications.
Waveguide arrays are the cornerstone of optical communication devices and systems. For example, optical switching fabrics usually comprise massive waveguide arrays. Also, many devices, such as a wavelength demultiplexer and an integrated optical phased array, often employ a dense waveguide array for output. On the other hand, a waveguide array can be considered a discrete lattice of waveguides, which lends itself to a wide range of fascinating scientific studies (e.g., Anderson localization of light and parity-time symmetry) pertaining to condensed matter physics. In the cross-section of a waveguide array, a periodic lattice of high-index waveguide cores are embedded in a low-index background, in analogy to a periodic lattice of atoms. This has inspired researchers to apply many concepts rooted in condensed matter physics to the study of waveguide lattices, creating new frontiers in optics. In both device research and scientific studies, waveguide lattices generally have a large pitch, ranging from a few microns to tens of microns. As such, the inter-coupling between waveguides can be weak, which helps to reduce crosstalk. The subwavelength or submicron regime of lattice pitch has not been deliberately explored thus far. In addition, a simple lattice has usually been assumed.